Steric and counterion effects on cationic surfactant self-assembly into micelles and liquid crystals

The roles of head-group size and counterion association on aggregate morphology in solution and lyotropic phases of cationic surfactants ( tetradecyl trimethyl-, tetradecyl triethyl-, and tetradecyl tripropylammonium) are examined, using salicylate as a strongly binding counterion. Larger head groups are found to inhibit the formation of low-curvature structures such as bilayers, and salicylate binding excludes spherical micelles, so that both effects tend to favour locally cylindrical aggregates. Interfacial probes and ion flotation show that the binding of salicylate is reduced by increasing head-group size. In addition, a novel demixing is observed with features similar to lower consolute behaviour of other cationic surfactant systems.